This invention relates to a method for preparing ferrierite; more particularly, the invention relates to a method of crystallizing ferrierite at relatively low temperatures from reaction mixtures having compositions within specified ranges.
Crystalline alumino-silicates such as ferrierite in naturally occurring and synthetic forms have been shown to have catalytic capabilities for hydrocarbon conversion and highly selective adsorption properties.
These crystalline materials exhibit ordered structures having passages, cavities, or pores of definite ranges of sizes, characteristic for each variety. Such materials have been referred to as "molecular sieves" since the dimensions of the pores are such as to preferentially adsorb molecules of certain dimensions while rejecting those of larger dimension.
Ferrierite is the preferred crystalline alumino-silicate product of the method of the present invention. It is a naturally occuring zeolite mineral having a composition varying somewhat with the particular source. A typical elemental composition has been reported (D. W. Breck, Zeolite Molecular Sieves, John Wiley & Sons, 1974, p219) to have a composition corresponding to the formula: Na.sub.1.5 Mg.sub.2 [(AlO.sub.2).sub.5.5 (SiO.sub.2).sub.30.5 ] . 18H.sub.2 O.
The prominent structural features of ferrierite have been found by X-ray crystal determination to be parallel channels in the alumino-silicate framework. These channels, which are roughly elliptical in cross section, are of two sizes: larger channels having major and minor axes of 5.5 and 4.3A respectively, and smaller parallel channels having major and minor axes of 4.8 and 3.4A, respectively. Generally speaking ferrierite adsorbs molecules small enough to penetrate the channels of its porous structure, such as e.g. lower normal paraffin hydrocarbons such as methane, ethane, and in it's hydrogen form larger molecules such as propane, n-hexane and n-octane. Both alkali metal and hydrogen forms reject branched chain and cyclic hydrocarbon molecules.
Synthesis of ferrierite has been reported by Coombs et al, Geochimica Cosmochimica Acta Vol 17 P53 et seq (1959); Barrer et al, Journal Chemical Society, 1964, p485 et seq; Hawkins, Materials Research Bulletin; Vol 2 p951 et seq (1967) and Kibby et al, Journal of Catalysis, Vol 35 p256 et seq (1974). A disadvantage of these procedures is that relatively high reaction temperatures of about 300.degree. to 400.degree.C are employed; these high temperatures generate high pressures and require expensive process equipment.
An element of the instant invention is the use of sodium and potassium salts of certain polybasic acids. The use of sodium salts of polybasic acids in the preparation of another aluminosilicate, mordenite, is known from U.S. Pat. No. 3,758,667.